Note: I can only discuss these as they relate to San Francisco, USA. A casual carpool is a non-prearranged carpool, usually initiated by the driver. Either there is a general meeting place where drivers and passengers meet up, or the driver will "cruise" the bus stops/BART stations looking for passengers. The advantage for drivers on the to-San Francisco drive are that if they can find at least two passengers, they qualify to drive in the carpool lane, which is a lot faster going over the Bay Bridge and other bridges, and they don't have to pay the bridge toll. For the riders, they get a much more comfortable ride, usually quicker, and it's free. Basically a win-win situation. Plus, you get to meet interesting people. I would love to do it again--there are some websites for such things, and I think there's some info at (carpool hotlines, etc.).

The unusual aspect of the casual carpool is that you are riding with strangers. I'm sure this would be a bad idea in many places, but apparently it works just fine here. The driver this morning was a wheelchair-bound 23-year-resident of the Bay Area, and the other passenger a young man from Zambia. We had a great time talking all the way into SF.